Navarro is acting as his own lawyer.
There is more than one case. The civil suit filed by Navarro to quash the subpoena is him, pro se (him being his own lawyer); but he may hire a criminal defense lawyer for the criminal case filed by the government.
NYT sez: "The charges against Mr. Navarro come at a politically sensitive moment: one week before the committee is poised to begin a series of high-profile hearings on its findings."